This invention relates to an optical receiver for an optical communications system a receiver for an optical communications system for transmitting coded optical signals, comprising a first optical detector unit preceded by a first periodic optical filter. The invention also relates and to a method for operating the communications system comprising the steps of coding the optical signal in a transmitter by means of a transmitter filter and decoding it and converting it to a first electric signal in a receiver by means of a first periodic optical filter and a first detector unit, respectively.
In a paper by L. Mxc3x6ller, xe2x80x9cAn Optical CDMA Method Based on Periodic Spectrum Encodingxe2x80x9d, presented at the Thirteenth Annual Conference on European Fibre Optic Communications and Networks, Brighton, England, 1995, pp. 178-181, an asynchronous transmission system is described which uses a CDMA (code-division multiple access) method based on periodic spectrum encoding. In that system, the code words used for gaining access are implemented by different transfer functions of periodic filters. The method uses a particular periodic spectral filtering technique for wideband optical signal sources. An information signal can be received and processed in a receiver only if the periodicities of the filter transfer functions of the transmitter and receiver match.
A disadvantage of this method is that the periodicities of the transmitter and receiver filter transfer functions must match with a high degree of accuracy (order of 10xe2x88x925, relative). This results in a complex, controlled filter arrangement to compensate for drifts of the transfer function of the receiver filter caused by, e.g., changes in temperature.
It is therefore an object of the present invention to provide a receiver for an optical communications system of the above kind in which less stringent requirements must be placed on the stability of the filter transfer function. A further object is to provide a method of transmitting signals in such a communications system.
The first-mentioned object is attained by a receiver for an optical communications system for transmitting coded optical signals, comprising a first optical detector unit preceded by a first periodic optical filter, wherein the receiver has a second periodic optical filter whose roundtrip time is shifted with respect to the roundtrip time of the first filter by approximately one quarter of the mean lightwave period of the received signal, an optical splitter via which the inputs of the first and second filters are connected to the input of the receiver, a second optical detector unit following the second periodic optical filter, two squarers, each following a respective one of the two optical detector units, for squaring the amplitudes of the respective signals applied to them, and an adder for adding the output signals of the two squarers.
This object is also attained by a receiver for an optical communications system for transmitting coded optical signals, comprising of transmitting coded optical signals in an optical communications system, comprising the steps of coding the optical signal in a transmitter by means of a transmitter filter and decoding it and converting it to a first electric signal in a receiver by means of a first periodic optical filter and a first detector unit, respectively, wherein prior to the decoding, the coded optical signal is split up by means of an optical splitter and fed to the first periodic optical filter and a second periodic optical filter whose roundtrip time is shifted with respect to the roundtrip time of the first optical filter by approximately one quarter of the mean lightwave period of the received signal, that the signal portion filtered in the second periodic optical filter is converted into a second electric signal by means of second detector unit, that the amplitudes of the first and second electric signals are squared by means of a first squarer and a second squarer, respectively, and that the two squared electric signals are added by means of an adder. an optical detector unit preceded by a periodic optical filter, wherein the receiver has a modulator (MOD) for modulating the transfer function of the filter (Rx) at a frequency greater than the bit rate of the received signals, and the further object is attained by a method of transmitting coded optical signals in an optical communications system, comprising the steps of coding the optical signal in a transmitter by means of a transmitter filter and decoding it and converting it to a first electric signal in a receiver by means of a first periodic optical filter and a first detector unit, respectively, wherein prior to the decoding, the coded optical signal is split up by means of an optical splitter and fed to the first periodic optical filter and a second periodic optical filter whose roundtrip time is shifted with respect to the roundtrip time of the first optical filter by approximately one quarter of the mean lightwave period of the received signal, that the signal portion filtered in the second periodic optical filter is converted into a second electric signal by means of second detector unit, that the amplitudes of the first and second electric signals are squared by means of a first squarer and a second squarer, respectively, and that the two squared electric signals are added by means of an adder.
Both alternative solutions are based on the idea to provide means for achieving a smaller decrease in received power when the receiver filter is detuned relative to a nominal transfer function, such that in case of such detuning, the received power will pass not through the tuning curve with numerous sharp minima and maxima, but only through the envelope of this curve.
One advantage of the invention is that the filter only needs to be stable within approximately xc2x11%, so that the filter stability requirement is reduced by at least two orders of magnitude.
Another advantage lies in the fact that in the arrangement according to the invention, no active control of the filter is necessary for stabilizing the filter transfer function. This reduces the complexity and cost of the arrangement.